Some A 3 B fcc alloys undergo a chemical order-disorder phase transition. During the ordering process, four types of domain appear within the bulk, separated by antiphase boundaries ͑APB's͒. As for chemical ordering in the presence of a surface, scanning tunneling microscopy and He diffraction studies consistently show a contrasting behavior between vicinals, where APB's emerge at the surface, and low-index surfaces, where no APB's emerge. These differences emphasize the role of the step density in surface chemical ordering.